This invention relates generally to an apparatus in which mechanical vibrations are damped, and more particularly to an apparatus comprising a member subject to vibration, and a damping mass which is mechanically coupled thereto. In the preferred embodiment, the present invention is utilized as a means to damp the vibration of a mounting post on which vibration-sensitive optical equipment is secured.
Generally, to withstand the added dynamic stresses that result from vibration, a vibrating member must be strengthened structurally, which increases the cost and weight of the member's design. To reduce the need for structural strengthening, vibration damping mechanisms are commonly utilized to minimize the amplitude and acceleration of a member's vibration. Such damping mechanisms are used in mechanical structures of virtually all types to reduce the various adverse effects of vibratory motion. Vibration is particularly undesirable in high precision devices such as mounting posts for optical equipment, where the amplitude of the displacement of the member due to vibration is significant in terms of the degree of precision sought to be achieved.
Optical equipment mounting posts are typically designed so that a number of posts can be rigidly secured by their bases to a common surface, allowing various optical instruments to be secured to the posts and spatially aligned to perform various optical techniques. For example, a pair of posts may be mounted on a tabletop, one post having a light source such as a laser secured to it by means of an adjustable collar, with the other post similarly having a mirror secured to it. The relative positions of the posts and the optical components secured thereon may be adjusted so that the light source strikes the mirror and is reflected to a precisely determined destination. However, since the mounting posts and the attached optical instruments behave as a single rigid body, any vibration of the posts will cause the optical instrument to vibrate as well. In the above example, if the post on which the mirror is secured begins to oscillate relative to the light source, due to vibration, the reflected light beam will also oscillate, undesirably diminishing the degree of precision to which the destination of the reflected light beam can be maintained.
Even in a laboratory environment, a mounting post and its related optical equipment are difficult to isolate from various ambient disturbances which may vibrationally excite the post and cause it to resonate. Examples of typical ambient disturbances include loud noises such as those caused by passing aircraft, ground vibrations caused by the passing of large motor vehicles or by the operation of construction equipment, or even the vibrations of transformers within electrical devices utilized in the laboratory.
Since even the most rigid mounting posts will, at their respective resonant frequencies, be subject to vibration of an amplitude significant enough to effect the desired precision of the optical components, previous mounting posts have been designed with an internally disposed damping apparatus. In Matthews, et al. (U.S. Pat. No. 4,050,665) a rigid, tubular mounting post is disclosed in which at least two dynamically damped masses are disposed. The damping masses are serially aligned along the longitudinal axis of the post, and each mass is spaced from the adjacent masses and the interior wall surface of the post only by a resilient washer. Damping systems with serially-aligned masses are also disclosed in Osburn (U.S. Pat. No. 2,960,189), Aggarwal (U.S. Pat. No. 3,559,512) and Aggarwal, et al. (U.S. Pat. No. 3,690,414).
Each mass in the damped mounted post shown in Matthews comprises a unique damping system, with its own resonance characteristics. The resonance characteristics of these damping systems are mainly a function of the weight and location of the mass, the damping and spring constants of the washer material, and the size of the washers. Each damping system is selected so that its unique resonance characteristics will be optimally "tuned" to a particular vibrational frequency of the post. When the damping system is "tuned", the mass resonates out of phase with the post, and thus absorbs the vibrational energy of the post. When a plurality of tuned damping systems are employed, a preferred pattern of vibrational frequencies of the posts can be damped, resulting in effectively broadening the damping characteristics of the overall system.
While the Matthews device provides adequate damping at certain frequencies, its performance is less than satisfactory at the first order or fundamental resonant frequency of the post. As is well known, for a given amount of excitation force, the post's vibrations will be the largest when the excitation force is at a resonant frequency, for example at the cantilever mode of the post. The cantilever mode refers to the fundamental resonant frequency of the post with one end of the post rigidly secured to a surface, as the post is arranged in operation. Moreover, in most optical equipment applications, the majority of harmful ambient vibratory excitation forces are at low-frequencies which tend to excite the lower frequency resonant modes of the post, such as the fundamental mode. Thus, the lowest resonant frequencies of the post are subject to the greatest amount of excitation force, causing the amplitude of the vibrations at these resonant frequencies to be much higher than at other frequencies. Therefore, it is particularly important to damp vibrations at these lower resonant frequencies.
Accordingly, a need exists for a broadband vibration damped apparatus which is particularly effective in damping resonant frequency vibrations which are caused by low-frequency ambient excitations.